Method and apparatus for receiving a global positioning system signal using a cellular acquisition signal

ABSTRACT

Method and apparatus for a GPS device that uses at least one cellular acquisition signal is described. More particularly, a GPS device is configured to receive at least one cellular acquisition signal for obtaining benefits associated with AGPS with only a small subset of AGPS circuitry to interact with a cell phone network. This facilitates use of GPS devices without subscription to a cell phone service provider, thus avoiding cellular subscription fees.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application contains subject matter that is related toco-pending U.S. patent applications, application Ser. No. 09/884,874,filed Jun. 19, 2001, application Ser. No. 09/875,809, filed Jun. 6,2001, and application Ser. No. 09/715,660, filed Oct. 9, 2000, each ofwhich is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to mobile wireless devices formobile location systems, and more particularly to Global PositioningSatellite (GPS) receivers with improved indoor penetration for personallocation systems.

[0004] 2. Description of the Related Art

[0005] With the advent of GPS, there has been a growing demand formobile devices that may be used to provide a person's or an object'slocation. Devices built using conventional GPS receivers have beendeveloped by a number of companies. However, these devices havesignificant limitations, one of which is indoor penetration.

[0006] To address the above limitation of conventional GPS receivers, acombination of mobile GPS receivers and cellular infrastructurecommunicating via wireless links has evolved. This combination oftechnologies, known as Assisted GPS (AGPS), combines a GPS receiver witha cellular handset. The cellular handset provides a two-way link forcommunicating positioning data (“aiding data”).

[0007] In particular, performance of a conventional GPS mobile device inindoor environments may be limited by ability of the GPS mobile deviceto decode a navigation data stream broadcast by each of a plurality ofsatellites. Among other components, each navigation data stream containsa satellite trajectory model having parameters describing a respectivesatellite's orbit and clock variation as a function of time. Thesatellite trajectory model in the navigation data stream is sometimesreferred to as “broadcast ephemeris.” GPS mobile devices traditionallyreceive and decode the navigation data stream to extract the broadcastephemeris, which is needed to compute position. However, asignal-to-noise ratio in indoor environments is often insufficient fornavigation data bit decoding of the broadcast ephemeris. Thus, anothermeans of ascertaining satellite orbit and clocks variations was needed.

[0008] In AGPS systems, the satellite orbit and clock variation, orinformation derived from these components, is provided to the GPS mobiledevice via a two-way cellular link. A two-way cellular link is used torequest and receive information on such satellites, and the AGPS serviceis conventionally available only to authorized subscribers to a cellularnetwork.

[0009] While AGPS offers improvements in indoor penetration, addition ofa cellular handset and a subscription to a wireless provider adds to thecost and power consumption of a GPS receiver. Cellular handsets containcomplex and costly components. For example, the cost of adding a cellphone alone to a GPS receiver may be prohibitive for GPS applicationswhere a phone would otherwise be an unnecessary addition, let alone theaddition of a subscription fee of a cellular provider. Moreover,cellular transmission consumes power.

[0010] Therefore, it would be desirable to provide a GPS mobile devicethat is comparable in cost to conventional GPS handheld devices but withthe indoor penetration benefits associated with AGPS handsets.

SUMMARY OF THE INVENTION

[0011] The present invention provides apparatus and method for obtainingbenefits associated with AGPS without requiring complete integration ofa GPS device with a cellular handset. Furthermore, the present inventionfacilitates a GPS handheld or mobile device configured to operatewithout subscription to a cell phone service provider, and thuseliminates fees for such subscription. An aspect of the presentinvention is a GPS handheld device that comprises a cellular acquisitionsignal receiver or front end. It will be appreciated that circuitryrequired to receive an acquisition signal comprises only a portion of acomplete cellular handset. Particularly, a transmitter portion forcommunicating with a basestation of a cellular network is not includedin the GPS handheld device. Furthermore, digital signal processor andapplication processor(s) configured for modulating, demodulating, voiceprocessing, call protocols, subscriber identification and the like areabsent in the GPS handheld device. The cellular acquisition signalreceiver allows the GPS handheld device to have an accurate time of dayand/or frequency reference, thus assisting in GPS signal acquisition andGPS position computation.

[0012] An aspect of the present invention is a method for receiving aGPS signal. More particularly, a frequency correction burst is obtainedfrom a cellular network. A frequency offset responsive to the frequencycorrection burst is determined, and a window of frequency searchresponsive to the frequency offset is determined for receiving the GPSsignal. This may be done without having to transmit a cellular signal tothe cellular network, and this may be done without having to have asubscription to the cellular network.

[0013] Another aspect of the present invention is a method for receivinga GPS signal to a GPS handheld device. More particularly, a timesynchronization burst is obtained from a cellular network. A timingoffset responsive to the time synchronization burst is determined, and atime of day responsive to the timing offset is determined for receivingthe GPS signal. This may be done without having to transmit a cellularsignal to the cellular network, and this may be done without having tohave a subscription to the cellular network.

[0014] Another aspect of the present invention is a method fordetermining position of a GPS handheld device in proximity to a cellularbasestation of a cellular network. More particularly, at least one oflocation information and identification information is obtained from thecellular basestation, and a position estimate of the GPS handheld deviceresponsive to the at least one of location information andidentification information is determined. This may be done withouthaving to transmit a cellular signal to the cellular network, and thismay be done without having to have fee-based access to the cellularnetwork.

[0015] Another aspect of the present invention is GPS mobile device.More particularly, the GPS mobile device comprises at least one antenna.The at least one antenna is coupled to a cellular acquisition signalfront end couple to receive a cellular acquisition signal. AGPS signalfront end is coupled to the at least one antenna to receive a GPSsignal. A GPS/cellular processor is coupled to the GPS front end and tothe cellular acquisition front end. The GPS/cellular processor isconfigured with a GPS baseband processor in communication with the GPSfront end and a cellular acquisition signal baseband processor incommunication with the cellular acquisition signal front end. Areference oscillator is coupled to the GPS/cellular processor. A generalpurpose processor is coupled to the cellular acquisition signal basebandprocessor and to the GPS baseband processor, and memory is coupled tothe general purpose processor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] It is to be noted, however, that the appended drawings illustrateonly typical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

[0017]FIG. 1 is a network diagram of an exemplary portion of anembodiment of a GPS and cellular network in accordance with one or moreaspects of the present invention.

[0018]FIG. 2 is a network diagram of an exemplary portion of anembodiment of a GPS and computer network in accordance with one or moreaspects of the present invention.

[0019]FIG. 3 is a flow diagram of an exemplary portion of an embodimentof a mobile or handset GPS unit receiving cellular acquisition signalsin accordance with one or more aspects of the present invention.

[0020]FIG. 4 is a chip-level block diagram of an exemplary portion of anembodiment of GPS unit in accordance with one or more aspects of thepresent invention.

[0021]FIG. 5 is a signal detection diagram of an exemplary embodiment ofa frequency and delay window in accordance with one or more aspects ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] The present invention relates to implementing a GPS system usinga GPS handheld configured for receiving acquisition signals from acellular provider without requiring integration of the GPS receiver witha two-way capable cellular handset. The present invention provides oneor more benefits conventionally associated AGPS while eliminating therequirement for obtaining aiding data from a cellular network.

[0023] An aspect of the present invention is a method for configuringthe GPS device to store satellite trajectory data that replacesassistance data provided in AGPS systems. In AGPS systems, receivingassistance data from a cellular network requires a two-way capablecellular handset to request and receive such assistance data. Moreover,such services are commonly available only to paying subscribers to thecellular network. In particular, in one aspect of the present invention,satellite trajectory models are stored in memory in a GPS mobile device.The source of the satellite trajectory models stored in memory may bebroadcast ephemeris, received and decoded when a GPS mobile device isoutside in at least a medium signal strength signal environment in whichnavigation data decoding is feasible. Alternatively, satellitetrajectory models may be supplied to the GPS mobile device through acomputer network connection. If the satellite trajectory models comprisebroadcast ephemeris, the satellite trajectory models may be valid forbetween approximately two and six hours. In another aspect of theinvention, long-term satellite trajectory models are used, which may bevalid for days. Once the satellite trajectory models are obtained andare in memory, the GPS mobile device may function indoors for the periodof validity of the long-term satellite trajectory models. Thiseliminates the need to more frequently obtained assistance data as withconventional AGPS.

[0024] Another aspect of the present invention is a method fordetermining time offset of a GPS mobile device. More particularly, acellular acquisition signal may comprise a time synchronization signalthat is received by the GPS mobile device, enabling the GPS mobiledevice to establish a time of day for applying satellite trajectorydata. In addition, the time synchronization signal, if sufficientlyprecise, may be used to establish a delay search window, decreasing thesearch time required for GPS signal acquisition. In addition, the timesynchronization signal may be used to align coherent averaging intervalswith GPS signal data bits to improve signal to noise ratio.

[0025] Another aspect of the present invention is a method fordetermining time offset of a GPS mobile device. More particularly, acellular acquisition signal can comprise a time synchronization signalthat is received at the GPS mobile device, enabling the GPS mobiledevice's receiver to establish a time of day for applying satellitetrajectory data. In addition, the time synchronization signal, ifsufficiently precise, may be used to establish a delay search window,decreasing search time required for GPS signal acquisition. In addition,the time synchronization signal may be used to align coherent averagingintervals with GPS signal data bits to improve signal-to-noise ratio.

[0026] Another aspect of the present invention is a GPS mobile devicecomprising: one or more antennas configured to receive cellularacquisition signals and GPS satellite signals; radio frequency (RF)front end circuitry for the GPS signals; RF front end circuitry for thecellular acquisition signals; a cellular acquisition signal basebandprocessor; a GPS signal baseband processor; a time keeping countercommon to the baseband processors; a reference oscillator coupled to thetime keeping counter, baseband processors and front end circuitry; aprocessor coupled to the baseband processors; and memory coupled to theprocessor. In some embodiments, the GPS mobile device may additionallycomprise a computer network-docking interface or a data modem or both.

[0027] In the following description, numerous specific details are setforth to provide a more thorough understanding of the present invention.However, it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails. In other instances, well-known features have not been describedin order to avoid obscuring the present invention.

[0028] Referring to FIG. 1, there is shown a network diagram of anexemplary portion of an embodiment of a GPS and cellular network inaccordance with one or more aspects of the present invention. Satelliteconstellation 11 comprises a plurality of satellites. For purposes ofillustration four satellites, namely, satellites 11-1, 11-2, 11-3 and11-4, are shown, though fewer or more satellites may be used. GPS device10 is configured to receive one or more satellite signals 12 fromsatellite broadcast. GPS device 10 is configured to receive satellitebroadcast signals 12 as a form of one-way communication. GPS device 10is configured to receive one or more cellular broadcast signals 14 fromcellular basestation 13. GPS device 10 is configured to receive cellularbroadcast signals 14 as a form of one-way communication. GPS device 10may be configured to operate to receive satellite information fromsatellite broadcast signals 12 or a computer network connection, asdescribed below in more detail, or both. Furthermore, especially whenoperating in indoor or other satellite signal-harsh environments, one ormore cellular acquisition signals 14 broadcast from communication tower13 is utilized by GPS device 10.

[0029] Referring to FIG. 2, there is shown a network diagram ofexemplary portion of an embodiment of a GPS and a computer network forobtaining satellite information, such as one or more satellitetrajectory models, in accordance with one or more aspects of the presentinvention. GPS mobile device 10 may be put in communication withcomputer 22. Computer 22 may be put in communication with computernetwork 21, which may form a portion of an intranet or the Internet.Network 21 may be put in communication with server 23. Server 23comprises or has access to database 27. Database 27 comprises one ormore satellite trajectory models 39, such as for respective satellites11 of FIG. 1. Accordingly, server 23 may be in communication with one ormore GPS receiver stations 27-1, 27-2, and 27-3 via network 21 forreceiving broadcast ephemeris comprising satellite trajectory models 39.GPS mobile device 10 may have one or more satellite trajectory models 39downloaded to it from server 23.

[0030] Alternatively, server 23 may be put in communication withpublicly switched telephone network (PSTN) 25 via network 21. PSTN 25may be put in communication with telephone 24 which may be put incommunication with GPS mobile device 10, or telephone 24. In thisembodiment, a phone number, such as a toll free number, may be dialed inorder to download one or more trajectory models to GPS mobile device 10.

[0031] Connection between mobile device 10 and server 23 may beestablished to refresh satellite trajectory models 39. At other times,this connection may be absent. For example, in field conditions lackingcomputer network 21 connectivity, GPS handheld 10 may obtain satelliteinformation from one or more satellite signals 12 shown in FIG. 1. Suchinformation is typically valid for approximately two to six hours fromtime of broadcast. Before the validity period ends, a GPS receivershould attain another valid broadcast of ephemeris information tocontinue to operate.

[0032] In another embodiment, satellite tracking data from GPS referencestations 27-1, 27-2, and 27-3 is used in server 23 to create long-termsatellite trajectory models 39, which may be valid for periods of up toapproximately one week. Orbit models and associated long-term orbittrajectory data are described in more detail in co-pending and relatedapplications entitled “LONG TERM EPHEMERIS” to James W. LaMance, CharlesAbraham and Frank van Diggelen, application Ser. No. 09/884,874, filedJun. 19, 2001, and “METHOD AND APPARATUS FOR GENERATING AND DISTRIBUTINGSATELLITE TRACKING” to James W. LaMance, Charles Abraham and Frank vanDiggelen, application Ser. No. 09/875,809, filed Jun. 6, 2001. In oneaspect of the present invention, long-term orbit trajectory models areused in GPS mobile device 10 to extend the period of validity ofsatellite trajectory models 39 provided by server 23. This increases theinterval over which GPS mobile device 10 may be used in conditionswherein computer network 21 is not readily accessible.

[0033] Referring to FIG. 3, there is shown a flow diagram of anexemplary portion of an embodiment of a mobile or handset GPS unitreceiving cellular acquisition signals in accordance with one or moreaspects of the present invention. Cellular basestation 13 broadcastsseveral cellular acquisition signals including frequency correctionsignal 31, time synchronization signal 32, timing message, such as aframe number, signal 33, and cell identification number signal 34A.Notably, time message signal 33 may be a separate signal or may be atime message 33 provided with time synchronization signal 32. In someembodiments of cellular basestation 13, an additional signal, namelycell location signal 34B, is provided. It should be understood that oneor more of these broadcast elements 31, 32, 33, 34A and 34B may bepresent or absent in particular cellular network implementations.Furthermore, in some cellular networks one or more of broadcast elements31, 32, 33, 34A and 34B may be combined into various combinations ofcomposite signals. In accordance with one or more aspects of the presentinvention, one or more of these signals 31, 32, 33, 34A and 34B may beutilized, whether individually, jointly, or in various combinations.

[0034] Conventionally, cellular acquisition signals are provided toenable, at least in part, a cellular handset to synchronize to acellular basestation, as a first step in establishing communication witha cellular network. In particular, in the first phase of establishingcommunication, the cellular handset monitors specific frequencies forthe acquisition signals. In accordance with one or more aspects of thepresent invention one or more cellular acquisition signal is received,but a GPS mobile device does not continue with the subsequent stepsneeded to establish two-way communication with the cellular network. Inparticular, in accordance with one or more aspects of the presentinvention, a GPS mobile device does not transmit any data or message orboth to the cellular network. Furthermore, a GPS mobile device mayreceive one or more acquisition signals without a cellular networksubscription.

[0035] Referring to FIG. 4, there is shown a block diagram of anexemplary portion of an embodiment of a mobile or handheld GPS 10 inaccordance with one or more aspects of the present invention. Withcontinuing reference to FIG. 4 and additional reference to FIG. 3, extracircuitry is added to a conventional GPS receiver to allow one or morecellular acquisition signals 31, 32, 33, 34A and 34B, collectively andsingly cellular acquisition signals 102, to be received. This includes asecond radio frequency (RF) tuner, namely cellular acquisition front end131 coupled to an additional antenna, namely antenna 111. One or morecellular acquisition signals 102 are received by antenna 111 andprovided from cellular acquisition front end 131 to cellular acquisitionsignal baseband 136. Cellular acquisition signal baseband 136 is used tolock and decode one or more cellular acquisition signals 102, forexample, using conventional digital processing well known in the designof cell phones. For cost considerations, cellular acquisition front end131 may be integrated into a conventional GPS front end 132, which iscoupled to GPS antenna 133, on a single RF semiconductor integratedcircuit 130. Moreover, to save cost, cellular acquisition signalbaseband 136 may be integrated with a conventional GPS baseband 137 on asingle digital signal processing semiconductor integrated circuit toprovide a GPS/cellular processor 135. Examples of such integratedcircuits include, but are not limited to, a digital signal processor(DSP). However, more than one integrated circuit may be used, forexample, a DSP and an application specific integrated circuit (ASIC),and a DSP and an FPGA. Accordingly, by providing integrated circuits 130and 135, only a marginal increment in cost is added to a conventionalGPS. Furthermore, other technologies such as radio frequency CMOS(complimentary-metal-oxide-semiconductor) allow integration of basebandprocessor functions and front end functions into a single ASIC. Includedin this marginal incremental cost are additional filters and an extraantenna, described in more detail with respect to FIG. 4. Moreover,because cellular acquisition signals 102 are relatively high in power, asimple antenna may be used in order to control costs even further.Alternatively, a single antenna 100 capable of receiving both GPS andcellular signals may be employed.

[0036] The nature of a frequency correction signal 31 varies dependingon the cellular network. In CDMA systems, frequency correction signal 31may comprise a pilot channel. The pilot channel is a common channel thatis broadcast over a cell coverage area. Conventionally, the pilotchannel uses a repeating pseudonoise (PN) sequence of 2¹⁵ chips.Multiple basestations transmit the same PN code but at different timingoffsets to avoid mutual interference. To detect the pilot channel, GPSmobile device 10 may scan a range of PN code offsets until energy isdetected, indicating a cellular basestation transmitter, using acellular acquisition front end 131 of FIG. 4. By phase or frequencylocking to a detected pilot signal, GPS mobile device 10 may measure afrequency offset 35A related to error in a reference oscillator of GPSmobile device 10, such as reference oscillator 138 of FIG. 4.Alternatively, GPS mobile device 10 may make an open loop measurement offrequency error of a pilot signal to determine frequency offset 35A.

[0037] In GSM systems, frequency correction signal 31 is a frequencycorrection burst transmitted periodically by a basestation on one ofseveral frequency channels assigned to the basestation. The frequencycorrection burst signal 31 is an unmodulated tone transmitted at aspecific offset from a carrier frequency of the channel. GPS mobiledevice 10 may make an open loop measurement of frequency error from thefrequency correction burst to determine frequency offset 35A.

[0038] As one or more of broadcast signals 31, 32, 33, 34A and 34B arewell-known, for example in General System Mobile (GSM) systems and CodeDomain Multiple Access (CDMA) systems, among other known cellularsystems, unnecessary details regarding such signals are not repeatedhere for purposes of clarity.

[0039] In an alternative embodiment, optional reference oscillatorsteering circuit 142 of FIG. 4 is used to provide reference oscillatorsteering 35B. In this embodiment, frequency correction signal 31 is usedin connection with steering circuit 142 (shown in FIG. 4) to maintainreference oscillator 138 (shown in FIG. 4) within its nominal operatingfrequency range. Steering circuit 142 may comprise a digital-to-analogconverter connected to a voltage control input of an oscillator 138(shown in FIG. 4). Frequency steering a reference oscillator based on afrequency correction signal is well known in cellular handsets and isgenerally a requirement to ensure that the handset transmitter isprecisely maintained at an assigned transmission frequency. However, asGPS mobile device 10 does not comprise a cellular signal transmitter,there is no requirement to maintain a transmission frequency.Furthermore, GPS devices, in contrast to AGPS devices, tend to operateoff a non-steered reference oscillator, such as an oscillator withoutvoltage control. Thus, a non-steered reference oscillator 138 (shown inFIG. 4) may be used along with frequency correction signal 31 toprovide, or compute such as with cellular acquisition baseband processor136 (shown in FIG. 4), frequency offset 35A. Frequency offset 35A isprovided to frequency and delay search window 41.

[0040] An example of a frequency and delay search window 45 for aparticular GPS satellite signal 11 is shown in FIG. 5. As will beunderstood to those familiar with the art, frequency and delay window 45comprises a two dimensional space of uncertain frequency on frequencywindow 501 axis and uncertain code delay on delay window 502 axis. Toacquire a GPS signal, a GPS receiver 10 (shown in FIG. 4) searchesfrequency and delay search windows 501 and 502, respectively for a GPSsignal. An exemplary GPS signal response 503 is shown in FIG. 5. GPSreceiver 10 (shown in FIG. 4) detects signal response 503 by scanningwith one or more search bins 504. If frequency and/or delay uncertaintyis large, this search can be time consuming. This is especially true inindoor environments where, in order to obtain needed signal-to-noiseratio enhancements, GPS receiver 10 (shown in FIG. 4) dwells for periodsof several seconds accumulating signal power before advancing search bin504. Thus, it is beneficial to keep frequency and delay search windows501 and 502 as small as possible, especially for indoor operation.

[0041] With continuing reference to FIG. 5 and renewed reference to FIG.1, frequency window 501 is a function of Doppler uncertainty (due to therelative motion of GPS device 10 with respect to GPS satellite 11) aswell as frequency uncertainty due to imprecision of reference oscillator138 (shown in FIG. 4) in GPS device 10. Frequency offset 35A (shown inFIG. 3) provides an accurate estimate of offset from a referenceoscillator 138 (shown in FIG. 4) as frequency correction signal 31 isconventionally transmitted at a precise frequency. Thus, contribution ofreference oscillator 138 uncertainty to the frequency window 501 may besubstantially reduced or eliminated with frequency offset 35A. It shouldbe noted that adjustment of frequency window 501 may occur using asoftware algorithm in program memory 145 (shown in FIG. 4), with nospecial purpose circuit for steering voltage or controlling frequency ofreference oscillator 138. Alternatively, offsetting of frequency window501 may be achieved by altering frequency of reference oscillator 138with reference oscillator steering circuit 142 (shown in FIG. 4).

[0042] With renewed reference to FIG. 3, time synchronization signal 32may be obtained by GPS mobile device 10 to determine time offset 36. Thenature of time synchronization signal 32 varies depending on thecellular network. In CDMA systems, time synchronization signal 32 cancomprise a synchronization channel. The synchronization channel is acommon channel that is broadcast over a cell coverage area. The pilotchannel and synchronization channel of a particular cellular basestationuse an identical PN sequence, such as a PN sequence of 2¹⁵ chips.Additionally, the synchronization channel is modulated with a particularWalsh code, allowing it to be separated from paging and traffic channelsusing different Walsh codes. The synchronization channel carries atiming message 33. Specifically, in CDMA, the synchronization channelcarries a message containing a pilot PN offset that identifies time ofday offset of such pilot channel.

[0043] With renewed reference to FIGS. 3 and 4, in a CDMA compatibleembodiment, GPS device 10 may first detect a pilot channel of a nearbybasestation 13 with cellular acquisition front end 131, then proceed todecode a synchronization channel being broadcast by the same basestation13. GPS device 10 achieves synchronization to such a pilot channel at aparticular timing offset of GPS device 10 local timekeeping counter 139.Shortly thereafter, GPS device 10 receives time message 33 containing atime of day offset of such a pilot channel. GPS device 10 uses timemessage 33, along with the timing offset of local timekeeping counter139, to compute time offset 36. Since, in CDMA, basestation 13 time ofday is synchronized to GPS time used by GPS satellites 11 (shown in FIG.1), time offset 36 provides an absolute offset between local timekeepingcounter 139 and GPS time.

[0044] In GSM systems, time synchronization signal 32 is asynchronization burst transmitted periodically by basestation 13 on oneof several frequency slots assigned to cellular basestation 13. Timesynchronization signal 32 contains a unique header, such as a knownsequence of bits, that identifies a starting point of a burst. Inaddition, time synchronization signal 32 carries a timing message 33that comprises, among other elements, a GSM time stamp associated withsuch synchronization burst. In a GSM compatible embodiment, GPS device10 receives time synchronization signal 32, and uses header informationtherefrom to identify a starting point of a synchronization bursttherein relative to local timekeeping counter 139. GPS device 10 usesthis information, combined with timing message 33, to compute timeoffset 36. In this manner time offset 36 provides an offset betweenlocal timekeeping counter 139 and GSM timing of basestation 13. In someGSM networks, GSM timing is not synchronized with GPS time. Therefore,time offset 36 does not provide an absolute time offset to GPS time.Time offset 36 may however be used beneficially as an indicator ofrelative time, as discussed below.

[0045] Time accuracy of time offset 36 will be dependent on the cellularnetwork implementation. In systems such as CDMA that incorporate GPStiming within the cellular network, there is a high degree of timingaccuracy. In other networks, for example GSM, relative timing of a timesynchronization burst may be good, but an unknown offset may exist toGPS time. Finally in some systems, a time indicator may be an absoluteindicator, but with limited accuracy, for example time coming from acomputer server 23 (shown in FIG. 2), in which time and date weremanually set.

[0046] Depending on accuracy, available time offset 36 may be employedfor several purposes within GPS device 10. If timing offset 36 hasprecision substantially better than one millisecond, precise timecomponent 41 of timing offset 36 may be incorporated into frequency anddelay search window 45. Specifically, with additional reference to FIG.5, it is well known that, in the general case when precise timing is notavailable, delay window 502 spans an entire period of C/A code,nominally one millisecond (C/A code conventionally refers to codesavailable for civilian applications). This is because timing of locallygenerated C/A code within GPS baseband processor 137 is arbitraryrelative to GPS signals 12 (shown in FIG. 1). However, if precise timecomponent 41 is available, locally generated C/A code can be timedrelative to GPS signals 12 (shown in FIG. 1). Specifically, GPS device10 uses timekeeping counter 139 that is common to a C/A code generatorwithin GPS baseband 137 and to cellular acquisition baseband 136. Thus,time offset 36, determined from a time synchronization signal 32 asdescribed above, can be used in conjunction with local timekeepingcounter 139 to program a starting point of locally generated coderelative to GPS timing. In this manner, an uncertainty component ofdelay window 502 caused by an unknown relative timing of locallygenerated code is substantially reduced or eliminated. A remaining delaywindow 502 component is delay uncertainty related to unknown pseudorangeand any error in precise time component 41. As discussed below, apseudorange may be estimated from satellite trajectory models and anestimate of position, such as a position information 34 b. Thus, ifprecise time component 41 is accurate to substantially less than onemillisecond, delay window 502 may be reduced to substantially less thanone millisecond.

[0047] With continuing reference to FIGS. 3, 4 and 5, reducing frequencywindow 501 as an aspect and delay window 502 as another aspect,substantially reduces the total number of search bins needed to overallcover two dimensional frequency and delay search window 45. Asmentioned, this enables GPS receiver 10, or more particularly GPSbaseband processor 137, to search more rapidly, and therefore reduce thetime needed to obtain GPS satellite signals 12 (shown in FIG. 1).Furthermore, a reduced search window provides GPS mobile device 10 anopportunity to dwell longer at each search bin. Longer dwells providesignal-to-noise ratio enhancements that can enable weak signal receptionindoors.

[0048] Optionally, time offset 36 is provided to coherent averaging 50.Coherent averaging 50 improves signal-to-noise ratio in each search binby averaging correlation results from several consecutive cycles of C/Acode. When coherent averaging, impact of 50 bps navigation data bits ona GPS signal is to be considered. Specifically, due to navigation databits, a GPS signal undergoes a potential 180 degree phase transitionevery 20 cycles of C/A code. For signal-to-noise ratio enhancement,coherent averaging is performed over twenty consecutive cycles of C/Acode comprising a single navigation data bit. Furthermore, to enhanceperformance this averaging process should be synchronous with navigationdata bit timing, otherwise changing data bits may partially defeat suchan averaging process. For this reason, it is desirable to achievesynchronization of coherent averaging 50 with navigation data bittiming. Navigation data bit timing is uniform for all satellites 11(shown in FIG. 1) and synchronized with GPS time.

[0049] Timing of a data bit arriving at GPS device 10 is a function oflocal timekeeping counter 139 as well as pseudorange delay between GPSdevice 10 and a satellite 11 (shown in FIG. 1). Precise time component41 establishes a relationship between local timekeeping counter 139 andGPS time. Thus, if pseudorange is estimated as described below, precisetime component 41 may be used in conjunction with local timekeepingcounter 139 to control start and stop times of coherent averaging 50 soas to make a coherent averaging interval coincident with incomingnavigation data bits.

[0050] Time of day 42 is the absolute component of time offset 36,converted to units of GPS time units. This conversion can take severalforms, for example, a conversion from a Julian data system, or someother timekeeping standard employed by the cellular network. Time of day42 may be utilized within GPS receiver 10 even in an application whereprecision of time offset 36 is not better than one millisecond, namely,when precise time component 41 cannot be generated. In particular, timeof day 42 provides a reference time for ascertaining satellite positions43. Specifically, time of day 42 provides a reference time for satellitetrajectory model 39. As satellites 11 move rapidly through the sky, itis preferable that time of day 42 be accurate, within approximately atleast ten milliseconds, so that errors in prediction of satellitepositions will be on the order of meters or less. If, however, time ofday 42 does not provide this level of accuracy, error in time of day 42may be solved for as part of a navigation solution. In the lattersituation, accuracy of time of day 42 is unimportant, and a roughestimate of time is sufficient such as the time provided by a server ora real time clock. An example of such a method is Time Free GPS, asdescribed in more detail in co-pending application entitle “METHOD ANDAPPARATUS FOR TIME-FREE PROCESSING OF GPS SIGNALS” to Frank vanDiggelen, application Ser. No. 09/715,660, filed Oct. 9, 2000.

[0051] If time offset 36 has an arbitrary relationship to GPS time, timeof day 42 will not be directly available. However, time offset 36 may bebeneficial as an indicator of relative time. For example, GPS mobiledevice 10 may determine an initial time of day by the conventionalmethod of decoding the time of week (TOW) portion of a navigation datastream. TOW can be used to determine a relationship between cellularnetwork time and GPS time. For example, time offset 36 may representoffset between GSM system time and GPS time of day. Once thisrelationship is established, it may remain constant for extended timeperiods as cellular basestations use precise oscillators to generatetheir timing signals. Thus, GPS mobile device 10 may use time offset 36to determine time of day 42 based on a previously determinedrelationship between a cellular network and GPS time. In this manner,GPS mobile device 10 may be able to obtain positions in indoor operatingenvironments, utilizing a time synchronization burst from timesynchronization signal 32 to ascertain time of day. Furthermore, GPSmobile device 10 can function without a battery powered real-time clockto maintain time.

[0052] Cellular base station 13 may provide a cell identification number34A. The details of this message vary with cellular network. Cellidentification number 34A may be used to look up location of cellularbasestation 13 in a lookup table 37 stored in memory of GPS mobiledevice 10. This will give an approximate or estimated position 38 of GPSmobile device 10, namely, GPS mobile device 10 will be within the sectorassociated with longitude and latitude of communication tower 13location. As sector sizes vary from rural, suburban and metropolitanarea networks, this position estimate 38 will vary accordingly dependingon location of cellular basestation 13 within one of the above-mentionedarea networks. In those instances where cellular base station 13 isconfigured to provide its cell location 35B, cell location lookup table37 may be avoided and an estimate of position 38 provided based on celllocation signal 34B.

[0053] Estimate of position 38 is provided for line of sight calculation40. Specifically, estimate of position 38 is combined with satellitepositions, velocities and clock estimates 43 to determine expectedpseudoranges and pseudorange rates 44, and unit vectors 49 between GPSdevice 10 and each GPS satellite 11 (shown in FIG. 1). Line of sightcalculation 40, pseudorange and pseudorange rates 44, unit vectors 49,and delay and frequency measurements 47, are sufficient for position,velocity, and time computation 48. The details of such computations arewell known and will not be repeated here for purposes of clarity.

[0054] Pseudo range and pseudorange rates 44 are provided to frequencyand delay search window 45. In particular, pseudorange rate provides anestimate of Doppler shift between GPS mobile device 10 and each GPSsatellite (shown in FIG. 1), allowing frequency window 501 to bedetermined. Similarly, pseudorange provides an estimate of timing delaybetween GPS mobile device 10 and each GPS satellite 11 (shown in FIG.1), facilitating determination of delay window 502. As mentioned above,pseudorange and pseudorange rate 44 are components of frequency anddelay search window 45, more particularly frequency uncertainty inreference oscillator 138 and time uncertainty of locally generated C/Acode tied to time keeping counter 139, both of which may besubstantially reduced by means of cellular acquisition signals 102.

[0055] Integrated circuit 135 may comprise a time keeping counter 139for providing clock signals to baseband processors 136 and 137. Areference oscillator 138 may be used to provide a determined frequencywithin a tolerance to timekeeping counter 139. A general purposeprocessor, such as a microprocessor, 141 is coupled to receiveinformation from acquisition signal baseband 136 to provide an output toGPS baseband 137, as described with reference to FIG. 3. Microprocessor141 is coupled to memory 146, which may comprise partitioned memory orindividual memories 144 and 145. For individual memories, program memory145 is used to store programming, as described with reference to FIG. 3,for using one or more cellular acquisition signals to provideinformation regarding satellite range and range rate. Accordingly,program memory may be a programmable, non-volatile memory, such as anEPROM, E²PROM, flash memory, and the like. Data memory 144 may be usedto temporarily store data for microprocessor 141. Accordingly, datamemory 144 may be a programmable volatile memory, such as DRAM, SRAM,and the like. Optionally, a docking station, data modem and/or networkinterface 143 may be coupled to microprocessor 141 for receiving one ormore satellite trajectory models. Optionally, a digital-to-analog (D/A)converter 142 may be coupled to microprocessor 141 to receive a digitalsignal of a frequency and convert it to an analog signal of the samefrequency for providing a steering voltage to reference oscillator 138.

[0056] It should be appreciated that the incremental circuitry in theGPS device to receive and utilize cellular acquisition signals isminimal. In particular, the scope and cost of this circuitry is far lessthan that of a complete cell phone, which would include transmissioncircuitry, digital signal processing circuitry, voice processingcircuitry, a protocol stack processor, and many other components. Thus,it is anticipated that a GPS system in accordance with one or moreaspects of the present invention may be manufactured with less cost thanthat to produce conventional AGPS system.

[0057] While the embodiments described herein have provided details forGSM and CDMA systems, it should be apparent that the invention can beemployed in all types of cellular networks including iDEN, TDMA, AMPS,GPRS, CDMA-2000 and other 2.5 networks, and W-CDMA and other 3Gnetworks. Furthermore, the invention can accept multiple types ofcellular acquisition signals in a single device. In particular cellularacquisition front end 131 and cellular acquisition baseband 136 may beconfigured to incorporate simultaneous or sequential processing ofsignals from multiple networks. This would further facilitate use of aGPS device 10 anywhere in the world, not just within a prescribedcoverage region, and accordingly it would be desirable to provide anability to receive and use a set of cellular network signals.

[0058] Though GPS satellites were described, it should be appreciatedthat one or more aspects of the present invention may be used withpseudolites, ground based transmitters that broadcast a PN code similarto a GPS signal. Accordingly, the term “satellites”, as used herein, isintended to include pseudolites and equivalents thereof. Moreover, theterm “satellite signals” or “GPS signals” is intended to includesatellite-like and GPS-like signals from pseudolites and equivalentsthereof. Furthermore, though a GPS system was described, it should beappreciated that one or more aspects of the present invention areequally applicable to similar satellite positioning systems, includingwithout limitation the Russian Glonass system.

[0059] While the foregoing is directed to the preferred embodiment ofthe present invention, other and further embodiments of the inventionmay be devised without departing from the basic scope thereof, and thescope thereof is determined by the claims that follow.

What is claimed:
 1. A method for receiving a Global Positioning System(GPS) signal, comprising: obtaining a frequency correction burst from acellular network; determining a frequency offset responsive to thefrequency correction burst; and determining a window of frequency searchresponsive to the frequency offset for receiving the GPS signal withouthaving to have a subscription to the cellular network.
 2. A method forreceiving a Global Positioning System (GPS) signal to a GPS handhelddevice, comprising: obtaining a time synchronization burst from acellular network; determining a timing offset responsive to the timesynchronization burst; and determining a time of day responsive to thetiming offset for receiving the GPS signal without having to have asubscription to the cellular network.
 3. The method of claim 2 furthercomprising determining at least one of a satellite position and asatellite velocity responsive to the time of day.
 4. The method of claim3 further comprising determining a window of delay search for receivingthe GPS signal responsive to the satellite position.
 5. The method ofclaim 3 further comprising determining a window of frequency search forreceiving the GPS signal responsive to the satellite velocity.
 6. Themethod of claim 3 further comprising computing position of the GPShandheld device responsive to the satellite position.
 7. The method ofclaim 3 further comprising computing velocity of the GPS handheld deviceresponsive to the satellite velocity.
 8. The method of claim 2 furthercomprising determining a window of delay search for receiving the GPSsignal responsive to the time of day.
 9. The method of claim 2 furthercomprising: receiving the GPS signal to the GPS handheld device; andcoherent averaging responsive to the time of day to synchronize tonavigation data bits.
 10. A method for determining position of a GlobalPositioning System (GPS) handheld device in proximity to a cellularbasestation of a cellular network, comprising: obtaining at least one oflocation information and identification information from the cellularbasestation; and determining a position estimate of the GPS handhelddevice responsive to the at least one of location information andidentification information without having to have fee-based access tothe cellular network.
 11. The method of claim 10 further comprisingcomputing a pseudorange responsive to the position estimate.
 12. Themethod of claim 11 further comprising determining a window of delaysearch for receiving GPS signals responsive to the pseudorange.
 13. Themethod of claim 11 further comprising synchronizing coherent averagingto navigation data bits responsive to the pseudorange.
 14. The method ofclaim 10 further comprising computing a pseudorange rate responsive tothe position estimate.
 15. The method of claim 14 further comprisingdetermining a window of frequency search responsive to the pseudorangerate for receiving a GPS signal.
 16. The method of claim 10 wherein theidentification information is a cell identifier.
 17. The method of claim16 further comprising providing a lookup table relating basestationlocations to cell identifiers, wherein the step of determining theposition estimate comprises using the cell identifier to obtain acellular basestation location of the cellular basestation.
 18. Themethod of claim 10 wherein the location information is a cellularbasestation location of the cellular basestation.
 19. A GlobalPositioning System (GPS) mobile device, comprising: at least oneantenna; a cellular acquisition signal front end couple to the at leastone antenna to receive a cellular acquisition signal; a GPS signal frontend coupled to the at least one antenna to receive a GPS signal; aGPS/cellular processor coupled to the GPS front end and to the cellularacquisition front end, the GPS/cellular processor is configured with aGPS baseband processor in electrical communication with the GPS frontend and a cellular acquisition signal baseband processor in electricalcommunication with the cellular acquisition signal front end; areference oscillator coupled to the GPS/cellular processor; a generalpurpose processor coupled to the cellular acquisition signal basebandprocessor and to the GPS baseband processor; and memory coupled to thegeneral-purpose processor.
 20. The GPS mobile device of claim 19 whereinthe GPS mobile device does not comprise a cellular transmitter.
 21. TheGPS mobile device of claim 19 wherein the GPS mobile device is notintegrated with a cellular handset.
 22. The GPS mobile device of claim19 wherein the cellular acquisition signal front end and GPS signalfront end are integrated in a single integrated circuit.
 23. The GPSmobile device of claim 19 wherein the cellular acquisition signalbaseband processor and the GPS baseband processor are integrated in asingle integrated circuit.
 24. The GPS mobile device of claim 19 furthercomprising a timekeeping counter coupled to the reference oscillator andin electrical communication with the cellular acquisition signalbaseband processor and the GPS baseband processor.
 25. A method forreceiving a Global Positioning System (GPS) signal, comprising:obtaining a frequency correction burst from a cellular network;determining a frequency offset responsive to the frequency correctionburst; and determining a window of frequency search responsive to thefrequency offset for receiving the GPS signal without having to transmita cellular signal to the cellular network.
 26. A method for receiving aGlobal Positioning System (GPS) signal to a GPS handheld device,comprising: obtaining a time synchronization burst from a cellularnetwork; determining a timing offset responsive to the timesynchronization burst; and determining a time of day responsive to thetiming offset for receiving the GPS signal without having to transmit acellular signal to the cellular network.
 27. The method of claim 26further comprising determining at least one of a satellite position anda satellite velocity responsive to the time of day.
 28. The method ofclaim 27 further comprising determining a window of delay search forreceiving the GPS signal responsive to the satellite position.
 29. Themethod of claim 27 further comprising determining a window of frequencysearch for receiving the GPS signal responsive to the satellitevelocity.
 30. The method of claim 27 further comprising computingposition of the GPS handheld device responsive to the satelliteposition.
 31. The method of claim 27 further comprising computingvelocity of the GPS handheld device responsive to the satellitevelocity.
 32. The method of claim 26 further comprising determining awindow of delay search for receiving the GPS signal responsive to thetime of day.
 33. The method of claim 26 further comprising: receivingthe GPS signal to the GPS handheld device; and coherent averagingresponsive to the time of day to synchronize to navigation data bits.34. A method for determining position of a Global Positioning System(GPS) handheld device in proximity to a cellular basestation of acellular network, comprising: obtaining at least one of locationinformation and identification information from the cellularbasestation; and determining a position estimate of the GPS handhelddevice responsive to the at least one of location information andidentification information without having to transmit a cellular signalto the cellular network.
 35. The method of claim 34 further comprisingcomputing a pseudorange responsive to the position estimate.
 36. Themethod of claim 35 further comprising determining a window of delaysearch for receiving GPS signals responsive to the pseudorange.
 37. Themethod of claim 35 further comprising synchronizing coherent averagingto navigation data bits responsive to the pseudorange.
 38. The method ofclaim 34 further comprising computing a pseudorange rate responsive tothe position estimate.
 39. The method of claim 38 further comprisingdetermining a window of frequency search responsive to the pseudorangerate for receiving a GPS signal.
 39. The method of claim 34 wherein theidentification information is a cell identifier.
 40. The method of claim39 further comprising providing a lookup table relating basestationlocations to cell identifiers wherein the step of determining theposition estimate comprises using the cell identifier to obtain acellular basestation location of the cellular basestation.
 41. Themethod of claim 34 wherein the location information is a cellularbasestation location of the cellular basestation.